1. Field of the Invention
The present invention relates to a semiconductor device having a function of detecting current flowing therethrough.
2. Description of the Prior Art
A conventional vertical MOSFET (metal oxide semiconductor field effect transistor) of what is called a double-diffusion type is shown in FIG. 1. In FIG. 1, an N.sup.+ -type silicon substrate 1b of a high impurity concentration has an upper surface on which an N.sup.- -type silicon substrate 1a of a low impurity concentration is laid to make up a drain. A P-type region 2 is diffused at a predetermined interval in a predetermined region in the N.sup.- -type silicon substrate 1a, and a source electrode 7 is electrically connected to the P-type region 2 thereby to diffuse an N.sup.+ -type region 3 of a comparatively high impurity concentration. Using as a channel 4 a portion around the surface of the P-type region 2 not formed with the N.sup.+ -type region 3, a gate electrode 6 of polycrystalline silicon or the like is formed at least on the channel 4 through an insulating film 5 made of SiO.sub.2 or the like. The gate electrode 6 is covered with an insulating film 11 formed by an oxidization. Further, the whole upper surface is covered by an interlayer insulating film 9. At the same time, the source electrode 7 made of an aluminum film or the like is formed on the surface of the N.sup.+ -type region 3, P-type region 2 and the interlayer insulating film 9.
In the prior art, the drain current or source current of the MOSFET (meaning operation current hereinafter referred to as "the drain current" uniformly) is detected by the voltage drop that occurs across a current-detecting resistor connected to the source electrode 7. The manner of detecting the drain current will be explained with reference to FIGS. 2 and 3. In an electrical circuit using the MOSFET as shown in FIG. 2, for example, the source terminal S is connected through a current detecting resistor 57 to a ground node g, and the drain current I.sub.D is detectable by a voltage drop V.sub.sg through the current detecting resistor 57. Specifically, the relationship ##EQU1## (where R.sub.57 is the resistance value of the current detecting resistor 57) is used to determine the drain current I.sub.D.
Now, the operation of the circuit in FIG. 2 will be explained with reference to the waveform diagram of FIG. 3. When a switch 51 is turned on and off in predetermined cycles, the applied gate voltage V.sub.G assumes the values of 0 and V.sub.10 alternately (FIG. 3(1)). When V.sub.G =0, the MOSFET 58 turns off, and when V.sub.G =V.sub.10, a conduction path is formed in the channel 4 to turn on the MOSFET 58. As a result of this on-off operation, the drain current I.sub.D flows through an inductance 54 and the current detecting resistor 57 as a load as shown in FIG. 3(2). The waveforms of the voltage drop V.sub.sg across the current detecting resistor 57 (FIG. 3(4)) and that of the drain current I.sub.D are similar to each other as seen from the proportional relationship indicated by equation (1), thus making it possible to detect the drain current I.sub.D. In FIG. 2, numeral 50 designates a DC power supply, and numeral 52 a resistor for reducing the gate applied voltage V.sub.G to zero when the switch 51 is open.
The conventional device using the current detecting resistor 57 shown in FIG. 2, however, has the disadvantages described below.
(1) The resistor 57 is required for detecting the current, increasing the size of the electronic device as well as the number of parts required, thereby leading to an increased cost.
(2) The current detecting resistor 57, which generates Joule's heat, is required to be cooled. (3) The voltage drop across the current detecting resistor 57 hampers the effective utilization of the power voltage.
In prior art methods, in order to prevent these disadvantages, the on resistance of the MOSFET without any current detecting resistor is used to detect the drain current I.sub.D from the drain-source voltage V.sub.DS. Specifically, the relationship ##EQU2## (where R.sub.DS is the on resistance, that is, the drain-source path resistance with the MOSFET turned on) is used to determine the drain current I.sub.D.
In FIG. 3, during the period when the drain-source path is in an on state (from t.sub.0 to t.sub.1, for instance), the waveform of the drain-source voltage V.sub.DS (FIG. 3(3)) is similar to that of the waveform of the drain current I.sub.D as shown by the proportional relations specified by equation (2), making it possible to detect the drain current I.sub.D.
Even this prior art method utilizing the on resistance has the problems mentioned below.
(1) As shown in FIG. 3(3), when the MOSFET 58 switches from on to off (such as at the time t.sub.1), a flyback pulse V.sub.P of high voltage is generated due to the inductance 54.
(2) As also shown in FIG. 3(3), when the MOSFET 58 turns off (such as at the time from t.sub.1 to t.sub.2), the voltage V.sub.21 of the DC power supply 55 is undesirably applied as the drain-source voltage V.sub.DS.
These problems prevent accurate detection since the drain-source voltage V.sub.DS does not become zero when the drain current I.sub.D is zero. Another problem is that generation of such a high-voltage flyback pulse V.sub.P damages the electrical circuit for detecting the voltage V.sub.DS in an extreme case.